Organic electroluminescent elements (organic EL elements) are viewed as the promising display elements of the next generation in that such elements operate at a voltage of as low as 10 volts or less, are capable of realizing various colors emitted by using organic compounds of different connective structures, and have high electroluminescent efficiency. In recent years, with the development of organic electroluminescent elements, their luminance and electroluminescent efficiency have become applicable to display devices.
Typical fabrication methods of such organic electroluminescent elements include a step of forming by the sputtering technique a transparent electrode (anode) having a flat surface above a substrate of glass or the like, a step of forming above the transparent electrode an emissive layer containing an organic electroluminescent material, and a step of forming by evaporation a metal electrode (cathode) such as magnesium and calcium above the emissive layer. In this fabrication method, if the electroluminescent material for the emissive layer is not a polymer (also referred to as a low-molecular electroluminescent material), the technique of evaporation is employed in forming the emissive layer. If, on the other hand, the electroluminescent material therefor is a polymer (also referred to as a polymer electroluminescent material), a solution containing the electroluminescent material is applied to form the emissive layer.
Conventionally, in organic electroluminescent elements, to increase luminance and electroluminescent efficiency, improvements have been made to the organic electroluminescent materials, or structural improvements have been made including provision of a function layer other than the electroluminescent material layer between the anode and cathode. The following describes an example of the lamination structure known in the art. Known emissive layers whose materials are polymeric and electroluminescent include those that have a two-layer structure of an electroluminescent material layer and a hole-transfer layer provided between the electroluminescent material layer and the anode. Known emissive layers whose materials are low-molecular and electroluminescent include those that have a three-layer structure of a electroluminescent material layer, a hole-transfer layer provided between the electroluminescent material layer and the anode, and an electron-transport layer provided between the electroluminescent material layer and the cathode. Other organic electroluminescent elements under development are those in which the emissive layer has a four-or-more-layer structure of function layers provided between the anode and cathode.
The principle of light emission of an organic EL element requires the injection of carriers (electrons and holes) (injection of current) into the emissive layer, which easily causes the decomposition of the organic compound composing the emissive layer and chemical or physical degradation of the boundary between the emissive layer and another layer. Thus, it is not easy to increase the lifetime of organic EL elements as compared with laser emitting diodes (LEDs), which employ inorganic materials for the emissive layer. The increase of luminance requires an increase in an operation voltage applied to the element. The greater the operation voltage is applied, the shorter the lifetime of the element becomes. Thus, it has been practically impossible to apply conventional organic electroluminescent materials and organic electroluminescent elements of conventional constructions to television, which requires high luminance, and to lighting systems, which require extremely high luminance.